Hydroforming, the use of fluid pressure to expand a tube or tube blank to comply with the shape of an enclosing die, is effective at providing tubular parts with a cross-sectional shape varying along their longitudinal axes. The cross-sectional shape of the resultant structural member is often appreciably different from the cross-sectional shape of the tube blank. Box shaped cross sections are commonly formed from cylindrical tube blanks. These cross-sectional shapes, however, will have a nearly constant wall thickness when the perimeter remains fairly constant along the length of the resultant structural member.
Conventional hydroforming, where the forming is essentially limited to pressurizing the tube blank to force it to conform to an enclosing die, produces little change in the length of a part. The length of the formed part is essentially equal to the length of the tube blank. For this reason, the cross-sectional area of the formed tube wall tends to remain constant along its length, equalling that of the tube blank. The constant area means that there is generally a tradeoff made between section perimeter and the local wall thickness. Hydroforming will generally increase perimeter and decrease wall thickness. It is appreciated that a variant form of hydroforming called bulge forming produces parts shorter than the tube blank to provide an increase in perimeter with little or no associated decrease in wall thickness. However, bulge forming requires a longitudinal loading of the blank simultaneous with the pressurization of the blank.
Permissible perimeter expansion of the tubing with conventional hydroforming is limited by the ductility of the tube material. More ductile material can sustain a greater amount of expansion.
It is often highly desirable to produce parts having wall perimeters and/or wall thicknesses which vary locally along the length of the formed part by more than the amount available with convention hydroforming, such as a tubular member with a localized 100 percent increase in wall thickness, or with a localized increase in perimeter of 100 percent. Unfortunately, conventional hydroforming does not readily lend itself to providing a part with such localized variations.
One approach to using conventional hydroforming to provide parts satisfying longitudinally varying thickness and perimeter requirements is to let the maximum perimeter and thickness dimensions control the dimensions of the tube blank for its entire length. Localized perimeter reductions are achieved by folding any excess tube perimeter into a flange on a side of the formed part. Letting the maximum dimensions control the tube blank dimensions, however, has an associated mass penalty because the resultant parts are heavier than they need to be.
Another approach is to hydroform the part in small sections from a variety of tube blanks having the locally desired characteristics, and then to weld the smaller hydroformed parts together. However, this carries a tooling cost penalty, as more die sets are required than if the final part is formed from a single blank in a single die set.
An alternative approach to providing locally thicker sections in the structural member is to use a tube of some nominal thickness to provide a base thickness, and use sleeves over the tube at those locations requiring a greater thickness to provide localized thickness increases. The sleeves are first placed over the tube blank. The tube and sleeves are placed in the die together for forming. Special fixturing is used to position the sleeves to axial positions on the tube. The expansion of the tube around the sleeve consumes a significant portion of the available amount of expansion of the tube, as the tube proximate to ends of the sleeve will be expanded to an outer perimeter approximately equal to that of the outer perimeter of the surrounding sleeve.